Resonance structures deactivators

It can be seen from resonance structures (2) to (4) that a — I — M-substituent deactivates the 3- and 5-position most strongly in electrophilic substitution. If this deactivation of the 5-position is strong enough to overcome the activating effects of the sulfur in the 5-position, substitution will be directed to the 4-position to an increasing extent. Tirouflet and Fournari studied the nitration of 2-substituted thiophenes of this type. The analysis was carried out polarographically, and the percentage of 4-isomer was as follows ... 

From resonance structure (12) it is obvious that a —I—M-substit-uent strongly deactivates the 2-position toward electrophilic substitution, and one would thus expect that monosubstitution occurs exclusively in the 5-position. This has also been found to be the case in the chlorination, bromination, and nitration of 3-thiophenecarboxylic acid. Upon chlorination and bromination a second halogen could be introduced in the 2-position, although further nitration of 5-nitro-3-thiopheneearboxylic acid could not be achieved. Similarly, 3-thiophene aldehyde has been nitrated to 5-nitro-3-thiophene aldehyde, and it is further claimed that 5-bromo-3-thiopheneboronic acid is obtained upon bromination of 3-thiopheneboronic acid. ... 

Illuminati et al. have also investigated the methoxydechlorination of 4-substituted-2- and 2-substituted-4-chloroquinolines. The relation between the reaction site, the 2- or 4-position, and the substituent in the 4- or 2-position, respectively, is always meta. The authors found the two reaction series well correlated with one another, but diverging quite seriously from the Hammett correlation. They concluded that mesomerically electron-donating substituents, because of the importance of resonance structures like 12 and 13, are more deactivating than expected, while electron-withdrawing substituents, and even the methyl group, seem to follow normal a correlation. 

The nitroso group, — N = Op is one of the few nonhalogens that is an ortho- and para-directing deactivator. Explain by drawing resonance structures of the carbocation intermediates in ortho, mela, and para electrophilic reaction on nitrosobenzene, C<3Hs N = 0. 

Now we can also understand why meta attack is preferred in a deactivated ring. Only if attack is at that position do none of the resonance structures of the transition state have a positive charge on that carbon that bears the electron-withdrawing group. 

However, if halogen atoms are deactivating the ring due to inductive effects, they should not direct substitution to the meta position like other electron-withdrawing groups. Consider the nitration of bromobenzene. There are three resonance structures for each of the three intermediates leading to these products, but the crucial ones to consider are those which position a positive charge next to the substituent. 

The reactant may be considered as a polarised carbonyl bond, reflected in contributions of covalent and ionic resonance structures. As the reaction proceeds, the contribution of both of these is replaced by the structure at the right of the diagram. Because of the greater concentration of positive charge on carbon in the reactant, donor substituents stabilise reactants more than transition state. In summary, donor substituents deactivate carbonyls. Cieplak acknowledged that the reactivity effect of a donor might be different from its stereochemical effect. 

Pyridine, like benzene, has six Jl-electrons. The electron-withdrawing nitrogen atom deactivates the ring, and electrophilic substitution is slower than that for benzene. Substitution occurs principally at the 3-position of the ring, as attack at the 2-/4-position produces less stable cation intermediates (i.e. with one resonance structure having a positive charge on the divalent nitrogen). 

On the other hand, the intermediates formed upon electrophilic substitution at the ortho or para positions on benzene rings with strongly deactivating substituents (such as nitro or trialkylammonio groups) involve contributions from resonance structures representing unstable carbocations. Such imstable resonance structures do not contribute to the resonance hybrid for the intermediate in the meta reaction (Figure 8.51). Therefore, the activation... 

These initial ideas were inadequate to explain the orientation effect of all groups. The problem appeared with the methyl group in toluene, which is activating, but for which no resonance structures could be constructed. The proper explanation of the regioselectivity (the preference for certain substituent positions) only came with the development of the theory of reaction mechanisms by Robinson and Ingold. As we have seen at the beginning of this chapter, the electrophilic attack causes the formation of a reaction intermediate, the carbocation. Since we know that the rate of chemical reaction depends mostly on the stability of the reaction intermediate, let us discuss the structure of the activated and deactivated cations. 

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